Scenario:
Eary in the game, Player X places a mosaic which creates a closed space, but not a sanctuary, as a tile still in the stack can be fit in that space. On a later turn, player Y places the last mosaic that can fit in this closed space elsewhere on the board, but not in this closed space. No other mosaic can now fit in this closed space created on the earlier turn. Can this player Y now claim this closed space for a sanctuary?

Rulebook says:
"If a player builds a Mosaic of the Gods inside the Palace's Gardens in such a way that a new, closed garden area is formed, in which no further Mosaic can be built [...] a Sanctuary is formed."

In your example, player Y isn't "forming" a Sanctuary, so he can't place an Anubi there.

What if the mosaics were being played by the same player on the same turn? The scenario would now be:

Player X purchases two mosaics, A (top of stack) & B (next in stack). Player X places mosaic A which creates a closed space, but not a sanctuary, as mosaic B, which has not been placed yet, can still fit in that space. Player X now places mosaic B elsewhere on the board, but not in this closed space. The closed space created by mosaic A is now a closed space which cannot be filled by any other mosaics. Can player X now claim this closed space for a sanctuary?

When is a closed space no longer "newly formed"? At the end of each mosaic placement or at the end of a turn?

The rulebook states:"If a newly formed Sanctuary is not immediately claimed by its rightful owner, no other player will be able to claim this particular sanctuary later in the game."

Does this imply that a player must claim newly closed spaces for sanctuaries after EACH mosaic laid or can he wait until the end of his turn to claim any sanctuaries created by any mosaics laid that turn?

If the player is allowed to play mosaic B before mosaic A, then the above scenario would be moot.

But if the following rule,"If the player chooses to build a Mosaic of the Gods, he must take the one on top of the pile in the Quarry..."
also implies that multiple mosaics purchased on the same turn must be played in stack order, then placing B before A would be illegal.

So, is the above scenario legal? How are these rules to be interpreted?

newly formed should be interpreted as "during the same player's turn" so what you describe is legal (and in effect, whether you place A then B or vice versa doesn't matter, as you correctly pointed out, as long as you place them during the same turn.

I understand the idea that you can claim an enclosed space you built earlier on your turn as a Sanctuary, if a Mosaic placed later on that turn now cause the enclosed space to be a Sanctuary.

However, I'm having trouble with the idea that two Mosaics purchased on the same turn can be played in any order. What you appear to be saying is the Quarry turn structure is as follows:

1. Play cards to purchase all the pieces that you intend to build this turn
2. Take all purchased pieces
3. Place purchased pieces in whatever order you like
4. Collect appropriate Talents for all pieces built

Let's say I purchase two Mosaics, A and B. What happens if, by building Mosaic B first, I now can't build Mosaic A?

It seems much cleaner if the Quarry turn order was:

1. Play cards to acquire one piece
2. Place that piece on the board
3. Repeat steps 1 and 2 until you're done building this turn
4. Collect appropriate Talents for all pieces built

This avoids any issues with the ability to acquire and play a specific Mosaic, because the "test" to see if the top Mosaic can be played is easier to administer if all remaining Mosaics are still in the pile. This also makes it much cleaner to score, as you know exactly what was in play when each piece was built.

The first "steps" you wrote, and were Eric's original answer, actuaully seem much cleaner and in line with the rules. Nothing states that you have to place the pieces in the order that you purchase them.

The outline you listed in the 2nd half does not jive with the rules: You cannot purchase a piece, then place it and then go back and purchase more pieces. The rules clearly state that you buy all pieces first, then place all purchased pieces.

The "complaint" about placing mosaic B first and then screwing yourself because mosaic A doesn't fit is not a logicial argument. Before you commit to purchase, you can always "touch" the merchandise. You know going in if Piece A and B can be installed and check it also dependent on the order placed. The rules allow you to look through the mosaic pile at any time through the game as long as you don't rearrange the pieces in the pile. So there is no may you wouldn't know that placing Piece B screws Piece A unless you were just a careless shopper.

You can use this strtegy to your advantaged. Maybe you could place A and B on the same turn if A goes first. But if you place B first, then you can't place A. Maybe you want this - pay the extra resources to get A just to discard it because what you really wanted was piece B that gives you the larger sanctuary now. Maybe getting to piece B empties the mosaics and move cleopatra.

I think the rules make perfect sense the way Eric stated them (and basically said the same thing as in the rule book). Any objections so far could easily be used as strategies to make a more effective turn.

"After building one (or more) Mosaic(s)," check the stack to see if the next Mosaic can be built.

Since the rules clearly state this "check" can occur after building multiple Mosaics, one should definitely purchase and build all pieces together. Then, and only then, should the Mosaic stack be checked.